1. Field of the Invention
This invention relates to a tire pressure monitoring system.
2. Description of the Related Art
An automobile (vehicle) operator conducting a pre-driving inspection can easily check the engine oil and coolant levels visually from the exterior but has no easy way to ascertain from the exterior whether the pressure of the tires is appropriate.
For overcoming this inconvenience, Japanese Laid-open Patent Publication No. Sho 43(1963)-17766 teaches a tire pressure monitoring system in which a pressure-actuated switch and a sensor unit comprising a small oscillator equipped with a transmitting antenna and other components is attached to each tire, the oscillator is operated to transmit an output when the tire pressure falls to or below a predetermined value, and the transmitted output is received by a receiving antenna-equipped monitoring unit installed near the operator's seat.
Moreover, well-known methods for detecting tire pressure include, in addition to the above, the method taught by Japanese Laid-open Patent Publication No. 2000-142043 of directly detecting tire pressure using a pressure sensor and the method taught by Japanese Laid-open Patent Publication No. Hei 6(1994)-92114 of estimating tire pressure from the output of an ABS (Antilock Brake System) wheel speed sensor.
When low tire pressure is ascertained by the foregoing conventional techniques, a warning to this effect is displayed on an indicator or the like provided on the dashboard near the operator's seat. The operator can then get out of the vehicle and inflate the tire whose pressure has become low. At this time, however, in order to reconfirm which tire requires more air before actually pumping air into the tire, the operator needs to turn on the ignition switch again so as to redisplay the information on the indicator.